The invention relates generally to transmission interlocks and more specifically to motor vehicle transmission interlocks which mechanically inhibit selection of certain gear ratio combinations.
Motor vehicle transmissions, especially those intended for use in trucks and tractors, often comprise plural multiple gear ratio selecting devices mechanically connected in tandem. For example, a five forward speed conventional transmission may be combined with an auxiliary two speed device utilized to drive the transmission countershaft. The resulting combination will provide ten forward speeds or gear ratios. The increased range of gear ratios provided by what is, in effect, a transmission having seven gear ratios is apparent. Less apparent but of equally great importance are the weight and space savings such a design exhibits over a conventional (non-tandem) transmission having for example ten independent and distinct gears and gear ratios.
In such a tandem transmission power train, it is frequently necessary to prevent operation of the transmission in a given combination of gear ratios due to intrinsic design or extrinsic application considerations. It thus may be necessary to positively prelude shifting such a transmission into certain combinations of gear ratios. For example, in a transmission having five forward gear ratios supplemented by a high/low range selection feature, it may be desirable to prevent simultaneous engagement of the fifth gear of the primary transmission and the high range of the auxiliary gear ratio selection device. Any means which will prohibit such a given selection must, of course, not effect the selection of any other gear ratio combination.
Interlock mechanisms which perform this function are known in the art. One such prior art mechanism employs a pneumatic logic system and actuator which inhibits a given attempted shift when a certain set of conditions exists. Since such systems are positioned externally of the transmission, they are subject to tampering by unauthorized personnel. These interlocks tend to be complex and their exposure to the hazards, both physical and chemical, of extended highway service may appreciably shorten their life.